1. Field of the Invention
The present invention relates to a semiconductor laser array provided with plural semiconductor laser elements monolithically formed on a semiconductor substrate. The semiconductor laser array of the present invention is particularly suitable for use as a light source in a multiple-beam scanning apparatus in which a recording member is scanned with plural beams for information recording, etc.
2. Related Background Art
As disclosed, for example in U.S. Pat. No. 4,571,021, when designing a light scanning apparatus by the use of a plurality of beam emitting devices such as laser diodes (LDs) or light-emitting diodes (LEDs), there has heretofore been known a method which comprises disposing the beam emitting devices so that the directions of emission of the lights from the beam emitting devices intersect one another at a point Po as shown in FIG. 1 of the accompanying drawings, and scanning a plurality of scanning spots relative, to a surface to be scanned while keeping a good image state.
FIG. 1 of the accompanying drawings shows a typical example of the prior art and is a view of the optical system between a light source and a deflector as seen from a direction perpendicular to a deflecting scanning plane. In FIG. 1, reference characters 71a and 71b designate beam emitting devices each comprising a laser diode. The devices 71a and 71b are disposed on a mount 72 as if the central rays ha and hb of the lights emitted from the devices 71a and 71b passed through the same point Po. In other words, if normals are drawn to the beam emitting surfaces of the respective devices, the normals are set so as to intersect each other at the point Po. Further, if seen from a direction parallel to the deflecting-scanning plane, the position at which the central rays ha and hb pass through the point Po is set so as to slightly deviate in a direction orthogonal to the deflecting-scanning plane. Also, said point Po and a point P near the deflecting-reflecting surface 73 of the deflector are kept in an optically conjugate relation by an imaging lens 74.
On the other hand, to obtain an effect similar to that shown in FIG. 1 where a monolithically formed laser diode array or the like is used as a light source, it is necessary to provide some optical system between the light source and the deflector. In the example disclosed in U.S. Pat. No. 4,565,421, a prism is disposed in front of a laser diode array. This is shown in FIG. 2 of the accompanying drawings.
FIG. 2 shows the cross-section of the prism in a case where the laser diode array has five light-emitting elements. In FIG. 2, reference numeral 81 designates the laser diode array having five light-emitting elements 81a, 81b, 81c, 81d and 81e, and reference numeral 82 denotes the prism. The central ray ha of the light beam from the light-emitting element 81a is refracted by an inclined surface 82a and bent as if it passed through the point Po. The central ray hb from the light-emitting element 81b, the central ray hd from the light-emitting element 81d and the central ray he from the light-emitting element 81e are bent by inclined surfaces 82b, 82d and 82e, respectively, as if they passed through the point Po. The central ray hc from the light-emitting element 81c passes perpendicularly through a flat surface 82c, and the point Po exists on the extension of this central ray hc. In this manner, there are provided inclined flat surfaces having their angles of inclination determined correspondingly to the respective light-emitting elements, and the central rays of the light beams after having emerged from the prism 82 have their directions controlled as if they were emitted from the point Po. This point Po, as previously described, is kept conjugate with a desired point P (not shown) near the deflecting-reflecting surface through an optical system.
On the other hand, FIG. 3 of the accompanying drawings shows an arrangement for providing a similar effect by a relay optical system 93. In FIG. 3, the relay system 93 is interposed between a collimator lens 92 for collimating and imaging lights emitted from the light-emitting elements 91a and 91b of a laser diode array and a cylindrical lens 95 to image the lights on the reflecting surface 94 of a rotational polygon mirror, and the lights are imaged on a surface to be scanned (not shown) in a good image state.
The problem in this case is the length of the optical path, that is, the length of the optical path of the relay system itself is longer by about 20 cm.
On the other hand, in order to resolve the above-mentioned drawbacks, the present applicant already proposed, in the U.S. application Ser. No. 797,492 (Filed on Nov. 13, 1985, still pending), a semiconductor laser array in which plural semiconductor lasers are formed as a monolithic array with respectively different light emitting directions.
FIG. 4 illustrates such as array, provided with respective semiconductor lasers 11-15, in which 11a-15a are current injection areas, namely light-emitting areas of said semiconductor lasers. Injection areas 11a-15a are formed in such a manner that the extensions thereof (hereinafter called resonant directions) 11b-15b respectively form angles .phi.a, .phi.b, .phi.c, .phi.d and .phi.e with a perpendicular line 18 to resonant planes 16, 17.
The light oscillated between the planes 16, 17 is bent approximately according to the Snell's law, as indicated by 11c-15c, when emerging through the plane 16 as a laser beam. Similarly the beams emerging from the plane 17 are emitted in directions respectively parallel to said directions 11c-15c. Consequently, in each semiconductor laser, the mutual angle of emerging light beams at one end of the array is same as that at the other end.
In such a semiconductor laser array with slanted beam emitting angles, the angles of the semiconductor lasers have to be large, and the design of the array becomes limited when the injection areas mutually cross as shown in FIG. 5 since the pitch of the lasers becomes smaller. More specifically the length Lc of the cavity requires a certain dimension for laser oscillation and is usually selected in the vicinity of 300 .mu.m. Thus, for a pitch of 20 .mu.m, the injection areas inevitably cross if the mutual angle of the injection areas is 3.degree.-4.degree. or larger.